Abstract: Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La{sub 1-x}Sr{sub 1+x}MnO{sub 4}(0{ =}0.45) doping. LaSrMnO{sub 4}(x=0) is an antiferromagnetic Mott insulator, and its spin-wave spectrum is well described by linear spin-wave theory for the spin-2 square-lattice Heisenberg Hamiltonian with Ising anisotropy. Upon doping, as the e{sub g} electron concentration (1-x) decreases, both the two-dimensional antiferromagnetic spin correlations in the paramagnetic phase and the low-temperature ordered moment decrease due to an increase of frustrating interactions, and Neel order disappears above x{sub c}=0.115(10). The magnetic frustration is closely related to changes in the e{sub g} orbital occupancies and the associated Jahn-Teller distortions. In the intermediate region, there exists neither long-range magnetic nor superstructural order. Short-range-correlated structural 'nanopatches' begin to form above x{approx}0.25. At high doping (x{>=}0.45), the ground state of La{sub 1-x}Sr{sub 1+x}MnO{sub 4} exhibits long-range superstructural order and a complex antiferromagnetic order, which differs frommore » that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites, and for x=0.50 we conclude that it is of B2mm symmetry. For x>0.50, the superstructural order becomes incommensurate with the lattice, with a modulation wave vector {epsilon} that depends linearly on the e{sub g} electron concentration: {epsilon}=2(1-x). On the other hand, the magnetic order remains commensurate, but loses its long-range coherence upon doping beyond x=0.50.« less

Abstract: Transport and magnetic properties have been studied in two sets of sol-gel prepared Pr1∕2Sr1∕2MnO3 nanoparticles having an average particle size of 30 and 45nm. Our measurements suggest that the formation of a charge-ordered state is largely affected due to lowering of the particle size, but the ferromagnetic transition temperature (TC) remains unaffected.

Abstract: We have studied the electronic structure and charge ordering (Verwey) transition of magnetite (Fe3O4) by soft X-ray photoemission. Due to the enhanced probing depth and the use of different surface preparations we are able to distinguish surface and volume effects in the spectra. The pseudogap behavior of the intrinsic spectra and its temperature dependence give evidence for the existence of strongly bound small polarons consistent with both dc and optical conductivity. Together with other recent structural and theoretical results our findings support a picture in which the Verwey transition contains elements of a cooperative Jahn-Teller effect, stabilized by local Coulomb interaction.

Abstract: Charge ordering phenomenon in $\theta$-(BEDT-TTF)$_2$X is studied by using 1/4-filled extended Hubbard model on an anisotropic triangular lattice through mean-field approximation. It is found that a metallic charge ordered state with 3-periodicity on lattices (3-fold type charge order) is realized in the realistic parameter region where the nearest neighbor Coulomb interaction V$_{ij}$ is nearly isotropic. This 3-fold state survives up to high temperatures by estimating the free energy. Insulating state with diagonal or vertical-type charge ordering appears as increasing anisotropy of V$_{ij}$. Considering the effect of the structural distortion observed in $\theta$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ at low temperatures, horizontal-type charge ordered insulating phase tends to be stabilized and the region where 3-fold type exists becomes narrower. Our results are consistent with the metallic state with long periodic charge order which can be related to 3-fold type in $\theta$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ at high temperatures and insulating state with horizontal charge order at low temperatures. For $\theta$-(BEDT-TTF)$_2$CsZn(SCN)$_4$, we speculate several kinds of charge-ordered states are energetically competing with each other leading to an inhomogeneous state at low temperatures.

Abstract: We report hole-doping dependence of the in-plane resistivity {rho}{sub ab} in a cuprate superconductor La{sub 2-x}Sr{sub x}CuO{sub 4}, carefully examined using a series of high-quality single crystals. Our detailed measurements find a tendency towards charge ordering at particular rational hole doping fractions of 1/16, 3/32, 1/8, and 3/16. This observation appears to suggest a specific form of charge order and is most consistent with the recent theoretical prediction of the checkerboard-type ordering of the Cooper pairs at rational doping fractions x = (2m + 1)/2{sup n}, with integers m and n.

Abstract: To investigate the possibility of charge order and superconductivity in a doped two-dimensional triangular lattice, we study an extended Hubbard model with the variational Monte Carlo method At n =2/3, a commensurate filling for a triangular lattice, it is shown that the nearest-neighbor Coulomb interaction V induces honeycomb-type charge order and antiferromagnetic spin order at \(U\gtrsim 10t\) We also discuss the possibility of superconductivity induced by charge fluctuation and the relation to the superconductivity in Na 035 CoO 2 ·13H 2 O and θ-type organic condoctors

Abstract: The effects of grain boundary morphology and stoichiometry had been systematically examined to clarify the role of natural grain boundaries in magnetoresistance of magnetite ${\mathrm{Fe}}_{3(1\ensuremath{-}\ensuremath{\delta})}{\mathrm{O}}_{4}$. We found that the excess resistance, caused by presence of the grain boundaries, is negligibly low in stoichiometric polycrystals. Accordingly, there was no grain boundary magnetoresistance detected in dense polycrystals. Moreover, the incorporation of grain boundaries was found to decrease the resistance of polycrystalline samples below the Verwey transition temperature. That was connected to the enhanced conductivity of grain boundaries appearing due to the local suppression of charge ordering. On the other hand, the essential negative magnetoresistance was detected in granular samples, exploring the point contact geometry for intergrain contacts. That magnetoresistance is characterized by large high-field component and appearance over a wide range of oxidation. It has been explained within the model of magnetically inhomogeneous grain boundary with the characteristic magnetic thickness of the order of exchange length. The magnetoresistance effect was connected to the spin-dependent scattering at the transition layers of magnetization formed around hard magnetic defects. The contraction of these transition layers by external magnetic field is supposed to provide the origin of the observed magnetoresistance. The analysis of appropriate microscopic scattering mechanisms reveals the important role of point defects in the spin-dependent scattering. The second magnetoresistance component was separated at highly oxidized grain boundaries and associated with tunneling transport across the isolating grain boundaries. Although the oxidation was shown to improve the isolating properties of natural grain boundaries, the performance of oxidized grain boundary as a tunneling barrier is still poor.

Abstract: A new π-d interaction system (EDT-TTFBr 2 ) 2 FeBr 4 (EDT-TTFBr 2 = 4,5-dibromo-4',5'-ethylenedithiotetrathiafulvalene) and its nonmagnetic anion analogue (EDT-TTFBr 2 ) 2 GaBr 4 based on a brominated TTF-type organic donor are investigated. The salts featured by quasi-1D π-electronic systems are metallic with metal-insulator transitions taking place at about 20 and 70 K for the FeBr 4 - and GaBr 4 - salts, respectively, where the low-temperature insulating state is associated with charge ordering or a Mott insulator followed by an antiferromagnetic transition at lower temperatures. The FeBr 4 - salt is featured with an antiferromagnetic transition of the anion d spins at a Neel temperature (T N ) = 11 K, which is significantly high despite its long anion-anion Br-Br contact, suggesting the importance of the π-d interaction in the magnetism. The surprisingly strong π-d interaction, ca. -22.3 K estimated from the magnetization curve, evidences the usefulness of the chemical modification of the donor molecule with bromine substitution to achieve strong intermolecular interaction. The antiferromagnetic state of the anion d spins affects the transport of the conducting π electrons through the strong π-d interaction, as evidenced by the presence of a resistivity anomaly of the FeBr 4 - salt at T N . Below T N , the FeBr 4 - salt shows negative magnetoresistance that reaches -23% at the highest magnetic field investigated (B = 15 T), whereas only a small positive magnetoresistance is observed in the π-electron-only GaBr 4 - salt. The mechanism of the negative magnetoresistance is explained by the stabilization of the insulating state of the π electrons by the periodic magnetic potential of the anion d spins in the FeBr 4 - salt, which is modified by applying the external magnetic field.

Abstract: A controversial issue of the driving force for the phase transition of the one-dimensional (1D) metallic In wires on Si(111) is studied by low-temperature scanning tunneling microscopy and spectroscopy. The energy gap opening and the longitudinal charge ordering through charge transfer at the Fermi level are unambiguously observed. The vacancy defects induce a local charge ordering decoupled from a lattice distortion above T(c), and pin the phase of charge order below T(c). All these results below and above T(c) including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.

Abstract: Polycrystalline samples of the charge ordered mixed oxide LaMn0.5Co0.5O3−δ (TCO = 400 K) have been prepared by the nitrate decomposition method. These samples are biphasic, according to XRPD, and except one, oxygen defficient (δ ≈ 0.04-0.05). The study of the dielectric properties of these samples reveal that LaMn0.5Co0.5O3−δ displays a high dielectric constant, specially at room temperature and low frequencies. This e′r is seen to be strongly dependent on the particle size and not so much on the oxygen defficiency and the best properties are found in the sample with biggest particle size (ϕ = 7 μm) for which e′r (300 K) ≈ 105 up to 2x104 Hz. Analysis of the role of the grain size and the electrode contacts on the obtained data reveal that this mixed oxide has an intrinsic dielectric constant that is rather high for this type of compounds (e′r,∞ ≈ 30) and that is further enhanced by extrinsic Maxwell-Wagner effects. We relate such enhanced intrinsic dielectric constant to the electronic process of charge ordering present in this material below 400 K.

Abstract: A systematic study of the electronic properties of Fe3O4 films grown directly on Si(001) substrates and on Ta, Ti, and SiO2 buffer layers using electron beam deposition is presented. The effect of the buffer layer on the Verwey transition temperature and on the current–voltage characteristics of Fe3O4 has been studied in detail. We observed that for a fixed Fe3O4 film thickness, the Verwey transition temperature is strongly dependent on the buffer layer materials. Transmission electron microscopy reveals that the growth mechanism of the Fe3O4 films is strongly dependent on the type of buffer layer used. The contribution of long range and short range charge ordering below the transition temperature has also been investigated. We observed an insulator-like gap structure in the density of states below the transition temperature which gradually disappears with increasing temperature.

Abstract: We study numerically the ground-state properties of the one-dimensional quarter-filled strongly correlated electronic system interacting antiferromagnetically with localized $S=1/2$ spins. It is shown that the charge-ordered state is significantly stabilized by the introduction of relatively small coupling with the localized spins. When the coupling becomes large the spin and charge degrees of freedom behave quite independently and the ferromagnetism is realized. Moreover, the coexistence of ferromagnetism with charge order is seen in the strongly interacting region. The present theoretical results are to be compared with the experiments on phthalocyanine compounds.

Abstract: Ultrathin La(0.8)Ca(0.2)MnO(3) films have been measured in a field-effect geometry. The gate electric field produces a significant ambipolar decrease in resistance at low temperatures. This is attributed to the development of a pseudogap in the density of states and the coupling of localized charge to strain. Within a mixed phase scenario, the gate effect and magnetoresistance are interpreted in the framework of a "general susceptibility," which describes how phase boundaries move through a hierarchical pinning landscape.

Abstract: We have performed electrical transport measurements at low temperatures and high magnetic fields in ${\mathrm{Na}}_{0.5}{\mathrm{CoO}}_{2}$ single crystals. Shubnikov--de Haas oscillations corresponding to only 1% of the area of the orthorhombic Brillouin zone were clearly observed, indicating that most of the original Fermi surface vanishes at the charge-ordering (CO) transition. In-plane magnetic fields were found to suppress strongly the CO state. For fields rotated within the conducting planes, we observe angular magnetoresistance oscillations whose periodicity changes from twofold to sixfold at the transition.

Abstract: ESR, NMR and X-ray measurements were performed for pristine and fully perdeuterio-TMTTF, TMTTF- d 12 salts. Significant enhancement by deuteration of the charge-order phase transition temperature, T CO , was observed in ESR measurements for all (TMTTF) 2 X salts measured. No obvious relation between the SbF 6 anion motion and the TMTTF charge-order was found by 19 F NMR. We also performed single crystal X-ray measurements to understand the deuteration effects and temperature dependence of the crystal structure. A possible relationship between the T CO 's and crystallographical parameters is proposed. The deuteration effects and possible origin of the charge-ordering transition of TMTTF salts are discussed.

Abstract: An analysis of the properties of half-doped manganites is presented. We build up the phase diagram of the system combining a realistic calculation of the electronic properties and a mean field treatment of the temperature effects. The electronic structure of the manganites are described with a double exchange model with cooperative Jahn-Teller phonons and antiferromagnetic coupling between the $Mn$ core spins. At zero temperature a variety of electronic phases as ferromagnetic (FM) charge ordered (CO) orbital ordered (OO), CE-CO-OO and FM metallic, are obtained. By raising the temperature the CE-CO-OO phase becomes paramagnetic (PM), but depending on the electron-phonon coupling and the exchange coupling the transition can be direct or trough intermediate states: a FM disorder metallic, a PM-CO-OO or a FM-CO-OO. We also discus the nature of the high temperature PM phase in the regime of finite electron phonon coupling. In this regime half of the oxygen octahedra surrounding the $Mn$ ions are distorted. In the weak coupling regime the octahedra are slightly deformed and only trap a small amount of electronic charge, rendering the system metallic consequentially. However in the strong coupling regime the octahedra are strongly distorted, the charge is fully localized in polarons and the system is insulator.

Abstract: The effect of Jahn-Teller (JT) interactions on specific heat and magnetic properties was studied for the series of La(1-x)Ca(x)MnO(3) (0.55 <= x <= 0.87) compounds by measurements of specific heat, ultrasonic velocity, and magnetization. Upon increasing x from 0.55 to 0.75, the enhancement of the JT interactions suppresses the ferromagnetic fluctuation above the charge ordering (CO) transition temperature T(CO), increases the change of entropy associated with the charge/orbital ordering transition, and therefore enhances the stability of the CO state. With further increasing x from 0.75 to 0.87, the ferromagnetic fluctuation gradually swells up and the change of entropy almost linearly decreases, due to the reduction of the JT interactions. The magnetic field dependence of T(CO) was estimated based on the Clausius-Clapeyron equation, which well coincides with that obtained from the transport measurements in high magnetic fields. These results imply that the JT effect is a key ingredient in understanding the essential physics of the CO state.

Abstract: The high-temperature structures and phase transitions of the cation-ordered manganite perovskites TbBaMn2O6 and YBaMn2O6 have been studied by powder neutron diffraction and differential scanning calorimetry. TbBaMn2O6 undergoes an unprecedented transition from a charge and antiferro-orbitally ordered phase to a charge and ferro-orbitally ordered structure at 94.5 °C, and the charge and orbital ordering melts at 200 °C. An orthorhombic Cmmm phase with rotational order of MnO6 octahedra is observed above the transition, and at 514 °C this transforms to the aristotype tetragonal P4/mmm arrangement. YBaMn2O6 is charge and ferro-orbitally ordered up to 225 °C. Above this charge ordering transition, a C2/m phase with two rotational orders is found. This transforms to the Cmmm structure at 437 °C. Discontinuities in volume and volume expansion, and a limited phase coexistence region are observed at the charge ordering transitions, whereas the other transitions are continuous.

Abstract: The magnetic, resistive, and structural properties of A-site layer-ordered La{sub 1-x}Ba{sub x}MnO{sub 3} materials (x=044, 048, 05, and 052) have been investigated Ferromagnetic properties have been observed for samples with x<05 Ferromagnetic and antiferromagnetic properties have been observed for samples with x{ge}05 For the x=05 sample, neutron powder diffraction data provide evidence for a structural phase separation below {approx}{approx}180 K and a stable and complex charge ordered (CO) phase at temperatures below {approx}120 K Contrary to previous reports, this charge ordered phase is not of the charge exchanged type and is not intrinsic to the main high temperature ferromagnetic phase The additional antiferromagnetic peaks observed in the neutron data indicate a quadrupling of the c axis (with respect to the primitive perovskite unit cell) for this charge ordered phase For the x=052 sample, a similar CO state has been observed within a narrow temperature range (at {approx}180 K) before it disappeared in favor of a more stable antiferromagnetic A-type orbital ordered phase The structures of all phases were refined in the tetragonal space group P4/mmm except for the A-type orbital ordered phase that assumes the monoclinic space group P2/m In this space group, the magnetic structure can be refined bymore » taking the time reversal of the two fold rotation axis leading to the magnetic space group P2{prime}/m« less

Abstract: A new mixed valent manganite CaMn4O8 with an original tunnel structure has been synthesized. The host lattice of this oxide has been determined by X-ray powder diffraction, using ab initio methods, and by high resolution electron microscopy. It consists of single and triple chains of edge-sharing MnO6 octahedra, interconnected through their apices. The [Mn4O8]∞ framework delimits three types of tunnels: six-sided (“H”) and eight-sided (“8”) where sit the Ca2+ cations, and empty rutile-like tunnels (“R”). The analysis of the Mn–O inter-atomic distances shows that this structure could also be described from the association of only single chains of edge-sharing MnO6 octahedra, with single chains of edge-sharing MnO5 pyramids, and evidences a high tendency of manganese to charge ordering. Nanostructural mechanisms investigated by electron microscopy also allow the structure to be described in terms of single bricks of four Mn polyhedra, and evidence twinning and intergrowth phenomena as well as local ordering of calcium in the tunnels. The magnetic measurements show strong antiferromagnetic fluctuations at low temperature, whereas the low value of TN ∼ 90 K indicates that the antiferromagnetic superexchange is weak in this oxide.

Abstract: We report the results of a low-temperature (300K–15K) high-pressure (up to 22GPa) Raman study of the Verwey transition in magnetite (Fe3O4). We use additional Raman modes observed below the Verwey transition to determine how the transition temperature changes with the quasihydrostatic pressure. Increase of the pressure results in the linear decrease of the Verwey transition temperature, with no discontinuity. The corresponding pressure coefficient dTV∕dP is found to be −5.16±1.19K∕GPa. Such a decrease is substantially larger than the one predicted by the mean-field Coulomb interaction model of the transition.

Abstract: High resolution X-ray structure analyses and electronic structure calculation revealed the condition for the charge ordering(CO) observed in the (TMTTF) 2 X (Fabre) salts. The phase diagram of the electronic states including 2k F and 4k F CO has been proposed associated with magnitudes of molecular dimerization and tetramerization.

Abstract: The surface electronic structure of 1500-A-thick Fe3O4(100) films has been investigated by spin-resolved photoemission spectroscopy with vacuum ultraviolet synchrotron radiation The films, epitaxially grown on MgO(100) substrates using plasma-assisted molecular-beam deposition, are briefly exposed to air during transfer to the photoemission chamber It is shown that clean surfaces of Fe3O4 films after exposure to air can be recovered through mild annealing in an oxygen atmosphere The presence of the characteristic Verwey transition, as detected in the valence-band photoemission spectra, provides evidence for the excellent electronic quality at the surface of these films The top of the valence band is found to be negatively spin polarized, with a value of ≈−50%; this result strongly points towards the localized nature of the 3d states in this system

Abstract: Charge ordering in the low-temperature triclinic structure of titanium oxide (Ti4O7) is investigated using the local density approximation (LDA)+U method. Although the total 3d charge separation is rather small, an orbital order parameter defined as the difference between t2g occupancies of Ti$^{3+}$ and Ti$^{4+}$ cations is large and gives direct evidence for charge ordering. Ti 4s and 4p states make a large contribution to the static "screening" of the total 3d charge difference. This effective charge screening leads to complete loss of the disproportionation between the charges at 3+ and 4+ Ti sites. The occupied t2g states of Ti$^{3+}$ cations are predominantly of $d_{xy}$ character and form a spin-singlet molecular orbital via strong direct antiferromagnetic exchange coupling between neighboring Ti(1) and Ti(3) sites, whereas the role of superexchange is found to be negligible.